Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 324,315	$ 306,392	$ 627,095	$ 602,687
Fixed-Price [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	136,178		258,825
Time-and-Materials [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	128,243		252,268
Cost-Based [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	59,894		116,002
U S Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	138,875		272,597
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	35,267		67,172
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	34,508		63,245
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	208,650		403,014
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	115,665		224,081
Energy, Environmental and Infrastructure [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	132,563		256,318
Health, Education and Social Programs [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	131,624		255,189
Safety and Security [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	27,491		52,927
Consumer and Financial [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 32,637		$ 62,661